There is a constant demand for the use of aluminum, plastic, and other lighter materials for automotive components in order to reduce the weight of these parts for the sake of improving fuel efficiency, reducing carbon dioxide emissions, and so forth. For example, the ceiling parts (roofs) of automobiles are generally made of iron, and attempts have been made to manufacture these roofs from aluminum. Meanwhile, because the body part to which the roof is mounted is made of iron, if an aluminum roof and an iron body part are brought into direct contact and bonded in place via rivets, nuts and bolts, or other mechanical means, the aluminum is corroded by electric corrosion occurring between the aluminum and the iron. Thus, when coupling aluminum and iron, it is necessary to dispose a spacer, such as a rubber sheet, between the aluminum and the iron to electrically insulate these materials.
One-pack thermoset adhesives, also known as structural adhesives, are used in the manufacture of automotive components. If such an adhesive is used in addition to rivets or nuts and bolts to couple a roof and a body part, the roof and the body part can be more strongly coupled. Because adhesives are ordinarily insulating in nature, the use of an adhesive to couple an aluminum roof and an iron body part in addition to mechanical joining will insulate these materials from each other, preventing corrosion of the aluminum due to electric corrosion. By applying an adhesive to the roof and/or body part, fixing the roof and body part in place using rivets or nuts and bolts, and subsequently performing electrodeposition and heating/drying, the heat from the heating/drying process can be used to cure the adhesive. However, when coupling the aluminum roof and the iron body part, differences in the linear expansion coefficients leads to the formation of gaps between the aluminum and the iron during the heating/drying process when a high temperature (for example, about 170-200° C.) is reached due to warping of these materials, thus forming spaces (voids) within the interior of the cured adhesive layer, or between the adhesive layer and the aluminum or iron. Moisture penetrating through voids between the aluminum and the iron carries the risk of corroding the aluminum and/or iron.
Japanese Unexamined Patent Application Publication No. 2002-284045A discloses a “method of joining automotive components made of heterogeneous metals, wherein an electrical insulating sealant which is adhesive to one mating surface of surfaces and is weakly- or non-adhesive to the other mating surface, is applied to the mating surfaces where one member and another member are joined, and the two components are mechanically joined”. Japanese Unexamined Patent Application Publication No. 2004-323639A discloses “a bonded structure for adherends exhibiting different levels of thermal expansion in which adherends exhibiting different levels of thermal expansion are bonded via an adhesive layer, wherein the adhesive layer has a two-layered structure, at least one layer of which comprises an epoxy resin having at least two epoxy groups as a primary ingredient and is formed from a flexible epoxy composition that is flexible when cured”.